A prior computer controllable multi-port valve is described in U.S. Pat. No. 4,156,437 issued on May 29, 1975. The patent lists Clyde C. Chivens and Wayne J. Whistler as inventors and is assigned to The Perkin-Elmer Corporation. The patent shows a rotating selector manifold for selecting between a plurality of input ports to interconnect with an output port. The selector manifold has a conduit for transferring gases and has resilient "O" rings to seal the conduit to the ports. The selector manifold is mounted on a shaft which rotates in a bearing attached to the housing of the multi-port valve. The patent also describes the use of a manifold position indicator to identify the position of the selector manifold.
Many problems are encountered in providing a system to collect and measure the composition of atmospheric gases from a plurality of remote locations using a mass spectrometer. Prior systems have been used to collect gases from remote locations, to pass the gases through a multi-port valve to select gases from a particular location for sampling, and to measure the gases repeatedly in sequence using the mass spectrometer. A multi-port selector valve used in such an application must be rugged in construction and avoid the use of components subject to wear, so as to avoid the need for frequent servicing. Since such gas measurement systems are often used in hospitals for patient monitoring or in industrial plants for environmental safety monitoring, it is very important that selector valves used in such systems operate continuously for long periods of time without failure or need for maintenance. Such selector valves should be inexpensive in construction and avoid the need for complex adjustments or alignment during manufacture or use. It is also important that such valves not produce false measurements when foreign materials, such as liquids exhaled by a medical patient, are mixed with the gases to be sampled. It is also important that the multi-port selector valves avoid contamination of the sample gases by preventing gas leaks into the valves.
Careful attention must be paid to avoid trapped volumes in valve and manifold systems in order not to degrade the rise time of the mass spectrometer sample measurement when gas composition changes occur. Also, it is important to keep the samples from remaining dormant in the line, and to reduce the delay time required to purge out the resident gases from the inlet lines when a sample is to be taken.